Method of decomposing raw phosphates



Patented May 28, 1935 UNITED STATES METHOD OF DECOMPOSING RA PHOSPHATES Sven Gunnar Nordengren, Landskrona, Sweden, assignor to Aktiebolaget Kemiska Patenter,

Landskrona, company Sweden, a Swedish joint-stock 7 No Drawing. Application September 23, 1931 Serial No. 564,720. In Sweden September 1,

9 Claims. (01. 23-165) It is lmown from the United States Letters Patent No. 1,776,595 to treat raw phosphates with sulphuric acid in such a manner that the calcium sulphate formed in the reaction is ohtained as semihydrate, CaSO4- A I-IzO, orwith 3' to permit washing or treatment with water, di

less water of hydration, or as anhydrite, or as a mixture of these bodies, and to separate it in this form or forms from the phosphoric acid. The object is to obtain the calcium sulphate in an easily separable form. t

It has now'been found. that the semihydrate as well as the anhydrite may be obtained in two forms, one of which is stable and the other unstable. The stable form is not subjected to changes when washed or treated with water, whereas the unstable form when treated in this wa takes up water and is converted into the dihydrate CaSO-l-2H2O, probably as follows;

In accordance herewith the present invention, which relates to decomposition of raw phosphates, apatite or other phosphoric acid-containing ma-v terial in the presence of the sulphuric acid radical, consists in subjecting the reaction mass during or after the decomposition by heating to such a solution vapor pressure that the calcium sulphate present therein becomes sufiiciently stable where p is the concentration of the phosphoric acid present at or after the reaction, expressed as percent by weight of P205 in the mother liquor, and t the temperature at the stabilization in degrees centigrade. In actual practice it may be preferable to use a somewhat higher temperature; for example 5-10 C; higher.

The following test is submitted to show the effect of the temperature on the stabilization of the calcium sulphate in the presence of phosphoric acid containing 33% P205. Semihydrate CaS0r /2H2O was treated with technically pure phosphoric acid of the said concentration at dif- 'EG ferent temperatures, whereafter the contents of water of hydration were determined. The separated precipitate was then treated with water during two hours at ordinary temperature and the percentage of water of hydration again determined:

It is pointed out thatthe temperatures were determined in the vapor space of the'vessel and that the calcium sulphate by contact with the walls of the vessel probably was heated to a somewhat higher temperature.

It will be seen that after heating to atemperature above 117 C. a suificiently stable semihydrate was obtained to permit treatment with water at ordinary temperatures during two hours without increase of the water of hydration. Such a temperature is however above the boiling point of the phosphoric acid and the stabilization must therefore in this case be carried out in a closed vessel. If, however, the concentration of the phosphoric acid is increased the stabilization may be carried out at a temperature corresponding to or lower than'the boiling point .of the phosphoric acid,i. e. in an openvessel. It has also been foundthat the degree of stabilization, which can be'expre'ssed in terms of the time the calcium sulphate canremain incontact' with water without substantial increase of the water of hydration, increases .withthe' concentration but that also the time of the treatment is of importance.

By means of tests such as that described above it should bepossible for anybody skilled in the art to find'in each case the conditions for obtaining a suificiently stable calcium sulphate. V It has further beenfound that if the raw phosphate is treated with more sulphuric acid than necessary for the decomposition and then more raw phosphate is added until'all the sulphuric acid is consumed, a stable calcium sulphate is more easily obtained, 1. e. at a lower temperature, than if the whole quantity of raw phosphate is added at once. h

It has also been found that in order to obtain in the decomposition of 'raw phosphates a phos-' sure a practically usable degree ofstabilization it should be greater than the quantityof the phosphoric acid formed in the reaction. This has to do with the vapor tension of the water of hydration of the calcium sulphate andthe vapor tension of the solution, as explained in the specification of Patent No. 1,776,595 and further with the necessity of having a certain quantity of water present in order tov ensure complete'or nearly complete dissolving out of the phosphoric acid, which in turn probably is due to-therfact that when insufncient water is present insoluble double salts between the calcium sulphate and the calcium compounds of the phosphoric acid are formed. These double salts are probably'intermediary products in the reaction between the tricalcium phosphate and the sulphuric acid and the solubility thereof may depend upon the fact that the compounds may crystallize with more or less water of hydration. When decomposing in open vessels the limit may probably be put at phosphoric acid containing at least 36 per cent by weight of P205; the quantity of the phosphoric acid added should be at least equal tothat of the phosphoric acid formed by the decomposition. I The phosphoric acid added may first be mixed with the raw phosphate or the phosphoric acid-containing raw material and brought to react therewith at ordinary or increased temperature, and the sulphuric acid then added.

The decomposition may be carried out with other acids than sulphuric acid but the sulphuric acid radical should be present or afterwards supplied to the reaction mixture.

A reaction mass which has been previously produced may also. be stabilized by heating with sulphuric acid and'then adding a corresponding quantity of raw phosphate; the sulphuric acid is then for determination of the required temperature calculated as the equivalent amount of phosphoric acid.

The phosphoric acid formed may directlyduring the reaction be converted into phosphate.

1. The process for decomposing raw materials containing phosphoric acid and calcium, which consists in treating said materials with'mineral acid in the presence of sulphuric acid'radical to produce decomposition, subjecting the reaction mass at the latest aftersaid inital decomposition to such a solution vapor pressure that the calcium sulphate produced will take up at most /2 .mol. of water of crystallization and thereby form stable crystals which fmay safely be subjected to washing with aqueous liquids and yet retain their content of water of crystallization unaltered in amount, by regulating the concentration of the phosphoric acid present in the mass and the temperature according to the formula 2P+t= at least 186 when P: concentration of the phosphoric acid expressed in percentage of weight of P205 of the mother liquor, and t the temperature in C.

2. The process according to claim 1, wherein there is added to the raw material at the latest during the decomposition thereof phosphoric acid from a preceding operation in a sufficiently high degree of concentration to conform to the conditions of the given formula.

3. The process according to claim 1, wherein there is added to the raw material at the latest during the decomposition thereof reaction mass from a preceding operation in a sufiiciently high degree of concentration to conform to the given formula.

4. The process'according to claim 1, wherein phosphoric acid is added to the reaction mass in a sufficient amount to contain more P205 than is newly formed from the raw materials being decomposed and the decomposition then carried out in the presence of such phosphoric acid.

5. The process for decomposing raw materials containing phosphoric acid and calcium, which consistsin first mixing said raw materials with phosphoric acid and causing the materials to react therewith, then treating said materials with mineral acid in the presence of sulphuric acid radical to produce decomposition, subjecting the reaction mass at the latest after said initial-decomposition to such a solution vapor pressure that the calcium sulphate produced will take up at most A.; mol. of water of crystallization and thereby form stable crystals which may safely be subjected to washing with aqueous liquids and yet retain their content of water of crystallization unaltered in amount, by regulating the concentration of the phosphoric acid present in the mass and the temperature according to the formula 2P+t= atleast l86. when P: concentration of the phosphoric acid expressed in percentage of weight of P205 of the mother liquor, and t: grees C.

6. The process for decomposing raw materials containing phosphoric acid and calcium, which consists in first treating solely a portion of the raw materials intended to be treated with mineral acid in the presence of sulphuric acid radical to produce decomposition, subjecting the resulting reaction mass at the latest after said initial decomposition to such a solution vapor pressure that the calcium sulphate produced will take up at most mol. of water of crystallization and thereby form stable crystals which may safely be subjected to washing with aqueous liquids and yet retain their content of water of crystallization unaltered in. amount, by regulating the concentration of the phosphoric acid present in the mass and the temperature according to the formula 2P+t= at least 186 when P: concentration of the phosphoric acid expressed in percen tage of weight of P205 of the mother liquor, and t: the temperature in degrees C. and thereafter adding the remainder of the raw materials to be treated to the reaction mass in order to again produce decomposition, and repeating the treatment of the reaction mass in respect to solution vapor pressure and regulation of the concentration of the phosphoric acid present in the mass as well as the temperature, according to said formula. v

7. The process for decomposing raw materials containing phosphoric acid and calcium, which consists in treating said materials with mineral acid other than sulphuric acid and sulphate radical to produce decomposition, subjecting the reaction mass at the latest after said inital decomposition to such a solution vapor pressure that the calcium sulphate produced will take up at the temperature in demost mol. of water of crystallization and thereby form stable crystals which may safely be subjected to washing with aqueous liquids and yet retain their content of water of crystallization unaltered in amount, by regulating the concentration of the phosphoric acid present in the mass and the temperature according to the formula 2P+t= at least 186 when P: concentration of P205 of the mother liquor, and t: the temperature in degrees C., and finally adding sulphuric acid radical to the mass.

8. The process for decomposing raw materials containing phosphoric acid and calcium, which consists in treating said materials with mineral acid other than sulphuric acid to produce decomposition, when said initial decomposition is complete adding sulphuric acid radical to the reaction mass, subjecting the reaction mass at the latest after the resulting decomposition to such a solution vapor pressure that the calcium sul- V phate produced will take up at most mol. of

water of crystallization and thereby form stable crystals which may safely be subjected to washing with aqueous liquids and yet retain their content of water of crystallization unaltered in amount, by regulating the concentration of the phosphoric acid present in the mass and the temperature according to the formula 2P+t= at least 186 when 1: concentration of the phosphoric acid expressed in percentage of weight of PzOs'of the mother liquor, and t: the tempera ture in degrees C.

9. The process according to claim 1, wherein the stable calcium sulphate associated with the phosphoric acid in the resultng mixture is retained therein and said mixture is then caused to react upon crude phosphate material.

SVEN GUNNAR NORDENGREN. 

